It is necessary to determine the position of motor-driven actuating parts, in particular in the case of closing parts such as window lifters, sunroofs or sliding doors in motor vehicles, in order to stop the closing part at predefined positions and in order to be able to satisfy the legal requirements in respect of anti-trapping protection. Generally, a position-determining process can be used in a very wide variety of actuating parts such as, for example, even in seats, and also in the case of blinds or sunshades, for example in order to be able to set previously determined or stored positions.
Such position-determining methods can be differentiated into two classes:
(1) On the one hand there are counting methods which are capable of precisely detecting the position always and under all given peripheral conditions. This is usually carried out by means of a magnet wheel attached to the motor shaft, in conjunction with two Hall sensors. The two Hall sensors permit the rotation of the motor to be determined and the rotational direction to be detected.
(2) On the other hand, there are position-determining methods or position-tracking methods which can determine the position but, under certain external circumstances, cannot avoid slight errors in the position-determining process in a system-immanent fashion. Such methods include 1-Hall position counters and, in particular, sensorless methods which infer the position from, for example, the current ripple of the commutator current of a DC motor. Such position-determining processes which are subject to errors make regular correction of the position necessary. This is carried out by means of devices for measuring positions, such as, for example, an initialization process.
A method which permits the current position to be tracked by means of relative changes in position is described in EP 2 102 725 B1, which is incorporated by reference. In this document, the current position is determined from the sum of all the changes in position on the basis of an initial absolute position determination, for example in a mechanically defined initialization process. Since every change in position is subject to uncertainty, the uncertainty of the absolute position calculated therefrom increases with the number of changes in position. Therefore, regular re-initialization by means of an absolute position-determining process is necessary in order to keep the uncertainty of the positions within acceptable limits.
DE 10 2007 050 173 B3, which is incorporated by reference, describes a method for correcting positions for a motor-driven actuating part by means of a position measurement which is carried out by means of a correlation function between a force/travel reference curve and a force/travel actual curve. In this context, the correspondence between the two curves is detected as a function of a position correction value between the curves, and the current position of the actuating part at a position corrected with the correction value determined in this way is assumed. Position information from other sources, such as, for example, an already present correction value, is, however, not taken into account or is used at most as a means of initializing the correlation search. Accordingly, possibly more precise position information, i.e. such information with a lower uncertainty value than the information determined from the correlation function, is also overwritten, and this information is therefore lost.